Over recent years, due to simple and inexpensive production of images, ink-jet recording systems have been used in a wide variety of printing fields including photograph, various types of printing, marking, and specialty printing such as color filters.
As ink-jet ink used in such ink-jet recording systems, there are various types of ink-jet inks such as aqueous ink containing water as a main solvent, oil-based ink mainly containing a non-volatile solvent which does not volatilize at mom temperature and containing substantially no water, non-aqueous ink mainly containing a solvent which volatiles at mom temperature and containing substantially no water, hot melt ink in which ink, being solid at room temperature, is heat-melted for printing, or actinic energy radiation curable ink-jet ink which is cured with actinic energy radiation such as light after printing. These inks are appropriately used based on the intended purposes. Of these, the actinic energy radiation curable ink-jet ink is characterized by being printed on various printing media due to rapid curability, having thereby received much attention as a next-generation ink-jet ink replacing the aqueous ink, the oil-based ink, and the non-aqueous ink requiring a large drying load and having a limited use of recording media. Therefore, such an actinic energy radiation curable ink-jet ink is an ink-jet ink whose use is expected to expand.
Conventionally, as the actinic energy radiation curable ink-jet ink, there are a radically polymerizable and a cationically polymerizable ink-jet inks. Further, of these, known are a non-solvent-type curable ink-jet ink containing substantially no solvent and a solvent dilution-type curable ink-jet ink of low viscosity positively diluted with water or a solvent. Fundamentally, the actinic energy radiation curable ink-jet ink features quick drying performance. Therefore, there has widely been brought into practical use a non-solvent-type curable ink-jet ink requiring no solvent-drying load during ink-jet recording or a curable ink-jet ink with a slightly added solvent. Of these, the radically polymerizable ink-jet ink features a wide selection of materials, which, thereby, has great flexibility for ink designing and is widely being researched and developed, resulting in being put into practical use. In contrast, the cationically polymerizable ink-jet ink is unaffected by polymerization inhibition caused by oxygen, and thereby has such advantages that excellent small droplet curability and excellent curability with a low energy radiation source are expressed and a relatively highly flexible cured film can be obtained.
Utilizing these advantages, application examples of such an actinic energy radiation curable ink-jet ink are cited as follows.
For example, with regard to printing of outdoor notice boards and advertising boards and printed matter of materials having a curved surface, a large-sized ink-jet printer is used. And as recording media, recording media formed of a plastic such as polyvinyl chloride or polyethylene are used. In these fields, long-term outdoor weather resistance, anti-abrasion properties, and solvent resistance are required. Further, use as rapping advertisement in buses or trains is occasionally done. In this case, printed matter is allowed to adhere to a vehicle body having a curved or an uneven surface. Therefore, a cured film is required to be flexible. As ink droplet size, a relatively large size is used due to large-sized printing, resulting in large film thickness. Therefore, when actinic energy radiation is inadequately transmitted into the interior of the film due to absorption by colorants such as pigments, poor curing occurs, resulting in impaired adhesion to the recording medium. Conventionally, in these fields, a non-aqueous ink-jet ink employing a pigment has been used. However, in use of such a non-aqueous ink-jet ink, printed matter is dried via evaporation of an organic solvent and penetration into the substrate, resulting in safety and odor problems. Accordingly, in recent years, an actinic energy radiation curable ink-jet ink is being applied utilizing its low odor properties and quick drying performance.
An actinic energy radiation curable ink-jet ink is applicable to printing for soft plastic packaging materials, utilizing the feature of printing ability with respect to a non-penetrable substrate. However, in this field, high definition, high image quality, and high productivity are required. Thereby, it is desirable to realize smaller droplets with respect to ink droplets ejected from the ink-jet head, to provide the ink with the ability to respond to thin film printing, and to achieve enhanced curability with no curing inhibition resulting from the ambience. Further, enhanced image quality having no problem of bleeding among droplets is required. Due to use for those held in the user's hand, safety and low odor are also required.
In the fields of various types of printed matter such as newspapers, magazines, books, or small-lot catalogs, a main technology is currently a conventional printing technology such as offset printing. An ink-jet recording system is being considered to be applied due to its features, specifically, such as cost performance in small-lot print run, delivery time shortening, or printability of variable data such as direct mail addressing. In these fields, problems such as printing speed, printing image quality, and drying load are noted. Therefore, the applicability of an actinic energy radiation curable ink-jet ink exhibiting quick drying performance and high ejection stability is expected. With the popularization of CTP (Computer to Plate) systems, plate images of a printing plate are also expected to be produced via an ink-jet system employing an actinic energy radiation curable ink-jet ink. Also in this case, required are realization of smaller droplet size, high image quality by a thinner film, reliability of ink-jet ejection, and printing durability.
As just described, with regard to an actinic energy radiation curable ink-jet ink, a fundamental technology having high general versatility is expected to be established to respond to a wide variety of demands such as various droplet sizes, recording media, printed matter size, printed film thickness, and various performing properties such as weather resistance, anti-abrasion properties, solvent resistance, flexibility, safety, odorless properties during printing, odorless properties of printed matter, or low running cost.
As a cationically polymerizable ink-jet ink composition, those, in which an oxetane compound, an alicyclic epoxy compound, or a vinyl ether compound is used as a polymerizable compound, are widely known (for example, refer to Patent Documents 1 and 2). In any of the ink-jet ink compositions disclosed therein, the added amount of an alicyclic epoxy compound is needed to increase to realize adequate curing sensitivity, which, thereby, has produced problems such as an increase in ink viscosity or a decrease in flexibility of a cured film. From the viewpoint of realizing ink viscosity reduction and flexibility of a cured film, a vinyl ether compound is a useful monomer due to its relatively low viscosity, as well as low glass transition point of a cured film. Therefore, actinic energy radiation curable ink-jet inks containing vinyl ethers are proposed and disclosed (for example, refer to Patent Documents 2-7).
In Patent Document 7, an image forming method which performs curing with an LED light source is disclosed in which an ink-jet ink containing a vinyl ether compound, a pigment, a disperser, a diallyl phthalate prepolymer, a cationic polymerization initiator, and a sensitizer are used. In the ink used herein, a vinyl ether compound of low viscosity is mainly contained, and also a prepolymer and a sensitizer are combined, whereby it is conceivable that the ink can exhibit lower viscosity, and excellent curing characteristics can be realized even using a long wavelength light source such as an LED. However, this type of ink is easily affected by humidity in air under the printing ambience. And when a light source of low illuminance such as a low pressure mercury lamp is used, under conditions where small droplets are singly arranged on a recording medium using an ink-jet system, effects, specifically, of humidity in air are easily produced, resulting in inadequate curing and poor drying which produce inadequate solvent resistance and anti-abrasion properties of a cured film. Thereby, a problem has been produced in which the light source used for curing is limited. Further, with regard to this ink-jet ink; when the ink is stored under a high temperature ambience, viscosity is increased, and in contrast, in the case of storage under a low temperature ambience, deposits tend to be generated, which has made it clear that a problem is noted in ink storage. Still further, when ink-jet ejection is repeated, the liquid repellency of the ink-jet nozzles is decreased, resulting in deposits near the nozzles, which has made it clear that there is also a problem in ejection stability.
On the other ham it is well known that in an ink-jet ink, especially, in an aqueous ink-jet ink employing water as a solvent, an inorganic salt such as sodium chloride causes corrosion of an ink-jet heat member and deposition of a colorant during ink storage. Further, it is known that even with regard to a non-aqueous ink-jet ink, chlorine ions contained in the ink induce corrosion of a liquid contact portion (for example, refer to Patent Document 8). Still further, a technology is disclosed in which in a cationically polymerizable composition, in order to ensure long-term storage stability with no inhibition of curability, the contents of cationic impurities, metal impurities, and strong-acid substances are specified (for example, refer to Patent Document 9). However, in the ink described in Patent Document 9, at present, poor curability and storage stability, specifically, poor liquid repellency of the ink-jet nozzles is expressed, and in addition, performance such as weather resistance or film flexibility is inadequately expressed.
In this manner, in an actinic energy radiation curable ink-jet ink, a highly robust ink-jet ink is being sought, in which curing thereof tends not to be affected by ambience factors (e.g., temperature, humidity, and oxygen) and by irradiation conditions (e.g., long time exposure with a low illuminance light source, short time irradiation with a high illuminance light source, and long wavelength exposure with an LED); and excellent curing can be carried out even using a small droplet size and a thin film and also excellent curing, reaching the interior of the film, can be performed even using a large droplet size and a thick film. Especially, it is critical that adverse effects by the humidity in air tend not to be produced under such conditions that ink-jet ink droplets are singly arranged on a recording medium at a low illuminance. In addition thereto, it is necessary to satisfy performing properties such as weather resistance, anti-abrasion properties, solvent resistance, flexibility, safety, odorless properties during printing, odorless properties of printed matter, low running cost, ink storage stability, and ejection stability.
[Patent Document 1] Japanese Patent Publication No. 3014251
[Patent Document 2] Japanese Patent Publication No. 3893833
[Patent Document 3] Japanese Patent Publication No. 3821912
[Patent Document 4] Japanese Patent Publication No. 4061876
[Patent Document 5] Japanese Patent Publication No. 4037856
[Patent Document 6] Unexamined Japanese Patent Application Publication (hereinafter referred to as JP-A) No. 2007-137923
[Patent Document 7] JP-A No. 2008-280460
[Patent Document 8] JP-A No. 10495356
[Patent Document 9] JP-A No. 2005-146001